1) Field of the Invention
The invention relates to a solid state laser pumped by a laser diode pump.
2) Description of the Related Art
Many scientific, medical, printing, ranging and industrial applications of small lasers require the laser to be reliable and efficient with high peak to average power ratio capability, and to be able to emit near diffraction-limited short-duration pulses in a controlled manner.
In U.S. Pat. No. 4,710,940 to Sipes (1987) and in U.S. Pat. No. 4,739,507 to Byer et al (1988) miniature continuous wave (CW) Nd:YAG solid state laser oscillators end-pumped by a discrete laser diode pump are disclosed. In these miniature lasers the output of a laser diode pump, typically having a power of 0.1-1W, is imaged or focused to a size similar to that of the TEM00 mode size of the solid state laser oscillator, that is to a focus which may be xcx9c50-200 xcexcm in diameter. In this way, the TEM00 mode (which gives the highest beam quality) is preferentially excited and caused to dominate the output of the solid state laser oscillator. Sipes teaches that, by concentrating the pump beam to a power density typically in the range 1-10 kW.cmxe2x88x922 within an oscillator crystal, the laser oscillator can be very efficient. In a paper entitled, xe2x80x98Diode Laser End Pumped Neodymium Lasers: The Road to Higher Powersxe2x80x99, (Proc Tuneable Solid-State Laser Conf, Paper TuC6, p134-6, May 1989, publ. Optical Society of America), Fields et al report achieving a laser diode pump to solid state laser optical power efficiency of up to 61% with this type of miniature laser oscillator using Nd:YVO4 as the oscillator material. U.S. Pat. No. 5,410,559 (1995) and U.S. Pat. 5,577,060 (1996) to Nighan et al teach that higher power performance can be achieved with larger lasers pumped at higher power where a oscillator is sufficiently long (typically 100 mm) to correspond to a large TEM00 mode size in an oscillator crystal, and where care is taken to mitigate beam degrading effects caused by a severe thermal load in the crystal oscillator.
Pulsed output can be achieved from miniature diode pumped laser oscillators. Microsecond laser pulses, typically in the range 1-500 xcexcs duration, may be achieved by using quasi-CW laser pump diodes ie diodes that can repetitively emit power for periods up to approximately 500 xcexcs. Nanosecond duration laser pulses can be achieved by adding a controlled optical Q-switch to a laser oscillator. Pulses shorter than 1 ns may be achieved by adding instead a passive Q-switch. In U.S. Pat. No. 4,761,786 to Baer (1988) the use of a miniature acousto-optic modulator as the Q-switch in a CW pumped laser is taught to allow use of a short solid state laser oscillator and to provide fast optical pulse dynamics. Baer teaches the production of pulses in the range 10-50 ns duration and of 10-20 xcexcJ energy from miniature oscillators using Nd:YAG and Nd:YLF as the laser crystal oscillators. In a paper entitled, Q-switching of a diode-pumped Nd:YVO4 Laser Using a Quadrupole E-O Deflector (Appl Phys B, Vol 67, p267-70, 1998), Friel et al report operating a short laser oscillator (around 15 mm long) and the production of 10-20 xcexcJ pulses of 1-2 ns duration and of the order of 10 kW peak power. If short pulses are required, and synchronisation is not important, a simple fast passive Q-switch (which can be very small) can be used and the oscillator made even shorter. This typically results in the generation of sub-nanosecond microjoule pulses at kHz repetition rate. In a review article entitled, xe2x80x9cQ-Switched Microchip Lasers Find Real-World Applicationxe2x80x9d. (Laser Focus World, August 1999, P129-36, PennWell Pub, USA), Zayhowski teaches that such lasers with a oscillator of only 0.75-1.5 mm length produce pulses of 0.2 ns duration and 141 xcexcJ pulse energy. The average output power was up to 120 mW with a maximum 1W laser diode pump power.
FIG. 1 illustrates a prior-art diode end-pumped miniature solid-state laser 100 including a Q-switched Nd:YAG solid state oscillator 110 that emits an output beam 150 at a wavelength of 1064 nm. In such a sold-state laser, a pump beam 121 from a discrete laser diode pump 120 operating at a pump wavelength of 808 nm is focused or imaged by lenses 131,132 onto an end face 112 of a Nd:YAG oscillator crystal 111 so that energy from the pump beam 121 is absorbed in the oscillator crystal 111 by exciting Nd ions. The crystal is typically a few millimetres in diameter and a few millimetres long. Stimulated emission of laser light occurs when the excited Nd ions are de-energised and the resultant light resonates in the oscillator by repeated reflections from the front face 112 of the crystal and a partially reflecting external mirror 115 to produce an oscillator beam 114 a proportion of which forms the output beam 150. To promote the reflections the crystal may have first and second high-damage-threshold dielectric coatings (not shown) applied to the face 112 illuminated by the pump diode and also to an opposed face 113 respectively. The first coating on face 112 is designed to transmit with low loss the laser diode pump beam 121 and to reflect the Nd:YAG oscillator beam 114, and the second coating on face 113 to transmit with low loss the oscillator beam 114. One or other or both of the reflecting surfaces comprising face 112 and that of mirror 115 may be curved to provide a stable oscillator. As illustrated, the laser oscillator 110 comprises in optical alignment, in addition to the miniature Nd:YAG crystal 111, and the partially reflecting output mirror 115, a miniature Q-switch 116 to allow the generation of laser pulses. The function of the Q-switch 116 is alternately to prevent and allow the oscillator 110 to resonate, so that while not resonating, increased energy is stored and on resonating, pulses of laser light are emitted.
When a quasi-CW pulsed pump diode 120 is used, the control of the diode and the Q-switch are synchronised. The laser output beam 150 exits the laser in the same direction as the pump diode beam 121 ie away from the diode 120. The principles of operation are well known to the art, and are, for example, described in xe2x80x98Solid-State Laser Engineeringxe2x80x99 by Koechner W, Springer Verlag, N.Y., Fifth Edition 1999, p363-370.
As taught by Sipes, the pump beam 121 at the crystal face 112 must be of a size similar to the TEM00 mode of the oscillator and have a power density of the order of 1-10 kWcmxe2x88x922 in the Nd:YAG crystal 111 to provide efficient operation. This restricts use of pump sources to discrete diodes. Since the work of Sipes, laser diodes have increased in CW power and diodes emitting 1-2W from a facet of approximately 100 xcexcmxc3x971 xcexcm are commercially available. Thus the output beam 150 of this type of miniature laser is typically in the average power range up to several hundred milliwatts.
The above prior-art arrangements are now widely used fairly efficiently and controllably to produce microjoule pulses of some kilowatts power from solid state lasers. However, a significant disadvantage of their miniature design is that they are not scalable to much higher average power, or greater pulse energy or peak power. This is because:
i) the oscillator must be short (the shorter the better) to provide fast laser pulse dynamics and a short pulse,
ii) the pump power must be low to avoid induced thermal distortion degrading the oscillator beam quality, and
iii) for good beam quality, the pump beam must selectively pump the active crystal oscillator only in a small volume close to that of the TEM00 mode.
As taught by Sipes, in a short oscillator, this last constraint requires high concentration of the pump beam for good performance. In particular, the constraints preclude use of higher power, larger area, laser diodes (or power diode arrays) because the output cannot be concentrated to a small spot corresponding to the TEM00 mode of the laser oscillator.
Laser diode bars (or arrays of such bars) 10 mm long suitable for pumping solid state lasers are commercially available operating up to a quasi-CW pulsed power of 100W, and CW power of 40W or more (see for example OptoPower Inc, Tucson Ariz., USA product HO1-D040-mmm-CS data sheet). Schemes have been disclosed readily to concentrate the output of such laser diode bars to spots of typically 1-0.5 mm diameter, but not 100-200 xcexcm diameter. Commercial devices are able to deliver 16W from a 20W-laser diode bar into a 0.6 mm spot with 0.37 numerical aperture (NA), but only of the order 1.2W power into a 100 xcexcm spot. (See for example SDL Inc, San Jose, Calif. data sheets on products SDL-3460-P6 and SDL-2372-P3, respectively).
As a consequence of these limitations and others, it has not hitherto been possible to scale short pulse miniature laser diode pumped pulsed lasers to much higher pulse energy and average power.
It is an object of the invention to at least partially mitigate these disadvantages.
In accordance with a first aspect of this invention there is provided a solid state laser comprising an optical oscillator, at least one optical amplifier, a laser diode pump arranged to provide a pump beam common to both the optical oscillator and the at least one optical amplifier, wherein a portion of the pump beam from the laser diode pump is arranged to pass through the optical amplifier before pumping the optical oscillator.
Conveniently, the optical oscillator and the at least one amplifier are arranged to be end-pumped by the laser diode pump.
Advantageously, the portion of the pump beam is focused by focusing means onto an end face of the oscillator and the focusing means is provided with an optical path for an oscillator beam to pass from the oscillator to an end face of the amplifier substantially unaffected by the focusing means.
Conveniently, the focusing means comprises at least one lens and the optical path comprises an axial bore through the at least one lens.
Conveniently, first reflecting means are included to reflect some of the portion of the pump beam emerging from the amplifier back into the amplifier.
Preferably, the first reflecting means is a spherical mirror having a axial aperture for the passage of the portion of the pump beam focused by the focusing means onto the oscillator means.
Advantageously, a polariser and a quarter wave plate are located between the focusing means and the optical amplifier such that a linearly polarised beam from the oscillator transmitted through the polariser and quarter wave plate is converted to circularly polarised light before entering the amplifier, and a beam emerging from the amplifier and passing a second time through the quarter wave plate is linearly polarised in a direction orthogonal to the original oscillator beam, to be reflected by the polariser out of the laser.
Conveniently, the optical amplifier has a truncated conical shape for relaying the portion of the pump beam onto the oscillator.
Preferably, second reflecting means are provided between the amplifier and the oscillator to reflect some of the portion of the pump beam back into the amplifier.
Advantageously, the optical oscillator and optical amplifier have a common optical axis.
Conveniently, the optical oscillator includes a Q-switch.
Preferably, active materials of the optical amplifier and the optical oscillator are rare earth doped crystals and in particular the active materials are selected from the group consisting of Nd:YAG, Nd:YVO4 and Nd:YLF.
Alternatively, active materials of the optical amplifier and the optical oscillator are rare earth doped glasses.
Conveniently, the active material of the optical amplifier is selected to be different from the active material of the optical oscillator.
Advantageously, the active materials are Nd:YVO4 and Nd:YAG.
Preferably, the laser diode pump is a CW laser.
Alternatively, the laser diode pump is a quasi-CW laser.
Conveniently, the laser is adapted to output high-power pulses.
Advantageously, the high-power pulses have an energy greater than 100 microjoules.
Conveniently, the laser is adapted to output pulses of less than 500 microsecond duration.
Preferably the output pulses are of less than 10 nanosecond duration.
Conveniently, the laser is adapted to produce an output beam substantially in a TEM00 mode.
In accordance with a second aspect of this invention there is provided a method of producing high energy pulses from a solid state laser comprising the steps of:
a) providing an optical oscillator and at least one optical amplifier;
b) providing a laser diode pump common to the optical oscillator and the at least one optical amplifier;
c) end pumping the at least one optical amplifier with the laser diode pump to produce a pumped optical amplifier;
d) end pumping the optical oscillator with the laser diode pump to cause the optical oscillator to emit a laser beam;
e) amplifying the laser beam with the pumped optical amplifier to produce an amplified laser beam; and
f) outputting the amplified laser beam from the solid state laser.
The optical amplifier medium is typically in the form of a small rod with a polished barrel, but may be of other design. In a preferred embodiment, the optical amplifier is end pumped by a high power array of laser diodes using optics that efficiently couple the laser diode output beam into substantially the full numerical aperture of the amplifier. An aperture in a stop plate between the laser diode and optical amplifier is dimensioned such that a cone including most of the laser diode pump beam impinges within the numerical aperture (NA) of the amplifier, which guides and absorbs power from the laser diode beam so coupled. A substantial proportion of the residual pump power that is transmitted through the amplifier is coupled to a miniature laser oscillator crystal, typically by beam coupling optics. By this means, sufficient power is delivered in a small volume to enable the laser oscillator to be pumped and to operate in a low order mode, preferentially in the TEM00 mode.
The laser oscillator is configured so that the output beam is emitted from the same end of the oscillator as that on which the incoming pump beam is incident. By this means, the emitted beam from the oscillator travels back through the amplifier in the contrary direction to the incoming pump beam and is amplified in so doing. Typically, the amplifier end faces (which may be planar or non-planar depending on the specific design) are anti-reflection coated at the output laser wavelength and the cylindrical barrel surface coated with an absorber at the output laser wavelength or other means used to mitigate parasitic oscillations and power loss. Conventional means are used to cool the amplifier via the barrel. A suitable choice of geometry and parameters allows the oscillator beam substantially to fill the amplifier aperture and efficiently extract stored energy absorbed from the pump beam. As a consequence, a high power pulsed output beam of high quality is emitted from the solid state laser. This beam is readily coupled out from the laser after amplification, eg by a dichroic mirror positioned between the diode pump source and the amplifier.